1. Field of the Invention
The present invention relates to an antenna device for UHF, VHF, FM etc., and having a base for extending and storing an antenna element.
2. Description of the Related Art
In general, it has been known to structure an antenna to be extendable (disclosed for example, Japanese Patent SHO 39(1964)-25826). This antenna will be explained with reference to FIG. 10 herein, where numerals 1 and 2 denote metal bands which are slightly arcuate in cross-section so as to be substantially rigid and straight under normal conditions. The metal band can be bent with a curvature when the sectional shape is flattened by a bending moment, and returns to its arcuate sectional shape to have a proper amount of flexibility and rigidity when the bending moment is removed. This type of metal band is commonly used for measuring tape devices. Numeral 3 is a guide piece having arms 4 and 5 extending diagonally upwardly on the upper portion thereof, and is provided with arms 6 and 7 in the lower portion thereof. The metal bands 1 and 2 have first ends fixedly connected to the arms 4 and 5 by screws 8 to extend diagonally upwardly. The bands form a curvature 9 at a certain lengthened position, extend vertically upward through the lower arms 6 and 7 and the inductive path formed in the guide piece, and have second ends connected to a movable control 10. In use, the metal band is extended and retracted by sliding the control 10 up and down to adjust the length of the projected portions extending diagonally upwardly by the sliding action. Therefore, the tuning frequency can be changed. The guide piece 3 is supported on a base 11.
The conventional device having the structure as described above, uses the process for adjusting the length of projected portion by moving upwardly or downwardly the control 10. Since the length of the metal band is already determined, the projected portion becomes smaller, but the vertical portion becomes contrary longer when the control 10 is moved upwardly for tuning in the radio wave having short wavelength, for example. In this case, since the antenna is fixed near to the earth in the electrical view, the adjustment of the height cannot be carried out. Further, when the radio wave having a short wavelength is tuned, it is difficult to keep the feeding point of the antenna at a certain amount because the vertical portion of the metal band acts as a trap circuit. Therefore, it causes a disadvantage that the electrical characteristic becomes unstable.
Further, when the vertical portion becomes longer, the center of gravity of whole antenna is at a higher position, thus causing the device to be unstable.